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autotroph

An organism that obtains organic food molecules without eating other organisms. Autotrophs use energy from the sun or from the oxidation of inorganic substances to make organic molecules from inorganic ones.

biogeochemical cycles

The various nutrient circuits, which involve both biotic and abiotic components of ecosystems.

biome

One of the world's major communities, classified according to the predominant vegetation and characterized by adaptations of organisms to that particular environment.

biosphere

The entire portion of Earth that is inhabited by life; the sum of all the planet's communities and ecosystems.

carnivore

An animal, such as a shark, hawk, or spider, that eats other animals.

community

All the organisms that inhabit a particular area; an assemblage of populations of different species living close enough together for potential interaction.

consumer, in ecological systems

A heterotroph that derives its energy from living or freshly killed organisms or parts thereof. Primary consumers are herbivores; higher-level consumers are carnivores.

decomposers

Saprotrophic fungi and bacteria that absorb nutrients from nonliving organic material such as corpses, fallen plant material, and the wastes of living organisms, and convert them into inorganic forms.

denitrification

The process by which certain bacteria living in poorly aerated soils break down nitrates, using the oxygen for their own respiration and releasing nitrogen back into the atmosphere.

detritivores

Organisms that live on dead and discarded organic matter; include large scavengers, smaller animals such as earthworms and some insects, as well as decomposers (fungi and bacteria).

ecological pyramid

A graphic representation of the quantitative relationships of numbers of organisms, biomass, or energy flow between the trophic levels of an ecosystem. Because large amounts of energy and biomass are dissipated at every trophic level, these diagrams nearly always take the form of pyramids.

ecology

The study of how organisms interact with their environments.

ecosystem

A level of ecological study that includes all the organisms in a given area as well as the abiotic factors with which they interact; a community and its physical environment.

food chain

The pathway along which food is transferred from trophic level to trophic level, beginning with producers.

food web

The elaborate, interconnected feeding relationships in an ecosystem.

herbivore

A heterotrophic animal that eats plants.

heterotroph

An organism that obtains organic food molecules by eating other organisms or their by-products.

nitrogen fixation

The assimilation of atmospheric nitrogen by certain prokaryotes into nitrogenous compounds that can be directly used by plants.

omnivore

A heterotrophic animal that consumes both meat and plant material.

photosynthesis

The conversion of light energy to chemical energy that is stored in glucose or other organic compounds; occurs in plants, algae, and certain prokaryotes.

population

A group of individuals of one species that live in a particular geographic area.

primary productivity

The rate at which light energy or inorganic chemical energy is converted to the chemical energy of organic compounds by autotrophs in an ecosystem.

producer, in ecological systems

An autotrophic organism, usually a photosynthesizer, that contributes to the net primary productivity of a community.

species pl. species

A particular kind of organism; members possess similar anatomical characteristics and have the ability to interbreed.

transpiration

The evaporative loss of water from a plant.

trophic level

The division of species in an ecosystem on the basis of their main nutritional source. The trophic level that ultimately supports all others consists of autotrophs, or primary producers.